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percpu_counter: extend _limited_add() to negative amounts

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Though tmpfs does not need it, percpu_counter_limited_add() can be twice
as useful if it works sensibly with negative amounts (subs) - typically
decrements towards a limit of 0 or nearby: as suggested by Dave Chinner.

And in the course of that reworking, skip the percpu counter sum if it is
already obvious that the limit would be passed: as suggested by Tim Chen.

Extend the comment above __percpu_counter_limited_add(), defining the
behaviour with positive and negative amounts, allowing negative limits,
but not bothering about overflow beyond S64_MAX.

Link: https://lkml.kernel.org/r/8f86083b-c452-95d4-365b-f16a2e4ebcd4@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Carlos Maiolino <cem@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2023-10-11 21:40:09 -07:00 committed by Andrew Morton
parent beb9868628
commit 1431996bf9
2 changed files with 49 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
include/linux/percpu_counter.h
View File

@ -198,14 +198,21 @@ static inline bool
percpu_counter_limited_add(struct percpu_counter *fbc, s64 limit, s64 amount)
{
unsigned long flags;
bool good = false;
s64 count;
if (amount == 0)
return true;
local_irq_save(flags);
count = fbc->count + amount;
if (count <= limit)
if ((amount > 0 && count <= limit) ||
(amount < 0 && count >= limit)) {
fbc->count = count;
good = true;
}
local_irq_restore(flags);
return count <= limit;
return good;
}
/* non-SMP percpu_counter_add_local is the same with percpu_counter_add */

48
lib/percpu_counter.c
View File

@ -279,8 +279,16 @@ int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)
EXPORT_SYMBOL(__percpu_counter_compare);
/*
* Compare counter, and add amount if the total is within limit.
* Return true if amount was added, false if it would exceed limit.
* Compare counter, and add amount if total is: less than or equal to limit if
* amount is positive, or greater than or equal to limit if amount is negative.
* Return true if amount is added, or false if total would be beyond the limit.
*
* Negative limit is allowed, but unusual.
* When negative amounts (subs) are given to percpu_counter_limited_add(),
* the limit would most naturally be 0 - but other limits are also allowed.
*
* Overflow beyond S64_MAX is not allowed for: counter, limit and amount
* are all assumed to be sane (far from S64_MIN and S64_MAX).
*/
bool __percpu_counter_limited_add(struct percpu_counter *fbc,
s64 limit, s64 amount, s32 batch)
@ -288,10 +296,10 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
s64 count;
s64 unknown;
unsigned long flags;
bool good;
bool good = false;
if (amount > limit)
return false;
if (amount == 0)
return true;
local_irq_save(flags);
unknown = batch * num_online_cpus();
@ -299,7 +307,8 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
/* Skip taking the lock when safe */
if (abs(count + amount) <= batch &&
fbc->count + unknown <= limit) {
((amount > 0 && fbc->count + unknown <= limit) ||
(amount < 0 && fbc->count - unknown >= limit))) {
this_cpu_add(*fbc->counters, amount);
local_irq_restore(flags);
return true;
@ -309,7 +318,19 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
count = fbc->count + amount;
/* Skip percpu_counter_sum() when safe */
if (count + unknown > limit) {
if (amount > 0) {
if (count - unknown > limit)
goto out;
if (count + unknown <= limit)
good = true;
} else {
if (count + unknown < limit)
goto out;
if (count - unknown >= limit)
good = true;
}
if (!good) {
s32 *pcount;
int cpu;
@ -317,15 +338,20 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
pcount = per_cpu_ptr(fbc->counters, cpu);
count += *pcount;
}
if (amount > 0) {
if (count > limit)
goto out;
} else {
if (count < limit)
goto out;
}
good = true;
}
good = count <= limit;
if (good) {
count = __this_cpu_read(*fbc->counters);
fbc->count += count + amount;
__this_cpu_sub(*fbc->counters, count);
}
out:
raw_spin_unlock(&fbc->lock);
local_irq_restore(flags);
return good;